OCN621: Biological Oceanography- Bioenergetics-II Guangyi Wang POST 103B guangyi@hawaii.edu http://www.soest.hawaii.edu/oceanography/zij/education/ocn621/ Chemosynthesis (Chemolithotrophy) Use of small inorganic molecules as an external energy source to power CO 2 reduction. Examples: 2 NH 4+ + 3 O 2 2 NO 2- + 4 H + + 2 H 2 O 2 NO 2- + O 2 2 NO 3-4F Fe 2+ + O 4F 2 + 4H + Fe 3+ + 2HO 2 HS - + 2 O 2 SO 2-4 + H + Shared characteristics: 1. Use energy from inorganic chemicals to generate ATP by electron transport phosphorylations with O 2 as terminal electron acceptor. 2. Use Calvin Cycle to fix CO 2 into glucose. 3. Reducing agent NADP has to be generated by utilization of some ATP not directly produced by the chemosynthetic process. 1
Anaerobic Chemosynthesis Some bacteria can live chemosynthetically without reducing O 2 and without assimilating CO 2 through the Calvin Cycle. Example: utilization of H 2 + CO 2 to produce methane 4 H 2 + CO 2 CH 4 + 2 H 2 O The energy derived from this process is used to reduce CO 2 to organics. Bacterial metabolism is exceptionally diverse. Many chemical substrates can serve as a source of energy for bacterial growth and production. Chemosynthesis (by bacteria) is generally not as important as photosynthesis in producing organic matter, but is clearly important in understanding elemental cycling in the oceans. 2
Definitions of nutritional modes AUTOTROPHIC - self-nourishing, organisms with the ability to synthesize organic molecules from CO 2. All photolithotrophic and chemolithotrophic organisms may be autotrophic, but many require small amounts of organic molecules - vitamins or essential amino acids which they cannot synthesize. These organisms are auxotrophic - requiring ii supplemental lnourishment. HETEROTROPHIC - depend entirely on organic molecules synthesized by other organisms. Osmotrophic heterotrophs which take up organic compounds by absorption through cell membrane. Phagotrophic heterotrophs which ingest particulate food. MIXOTROPHIC - organisms with mixed mode of nutrition. Some bacteria (chemoorganotrophs) use energy from small organic molecules to reduce CO 2 to sugars. Other mixotrophic organisms (e.g., protozoans) consume particulate food, but also contain functional chloroplasts or endosymbionts. Note: Both autotrophic and heterotrophic organisms manage to get small organic molecules into their cells, but they require additional energy to do anything with them. All of energy from sunlight is used up in photosynthesis; assimilation by heterotrophs requires (does not yield) energy. In order to live, all organisms have to convert small organic molecules into chemical energy which can then be used to do work. Extracting Energy from Organic Molecules GLYCOLYSIS - a "fermentation" reaction anaerobic decomposition of organics into "waste" product, lactic acid. The process involves about 11 steps & does not require a rigid organizational framework. C 6 H 12 O 6 2C 3 H 4 O 3 2C 3 H 6 O 3 glucose pyruvate lactate 2ADP+2Pj 2ATP 2NAD ox 2NAD re 2NAD re 2NAD ox If muscle tissue operates anaerobically (glycolysis) it builds up lactic acid and an oxygen deficit RESPIRATION - aerobic breakdown of food molecules yielding ATP. All basic organic constituents (sugars, fatty acids, amino acids) can be broken down in respiration, which begins where gylcolysis leaves off. Complex reaction system >100+ steps organized in mitochondria (cell "power plant") KREBS CYCLE (aka: Citric Acid or Tricarboxylic Acid Cycle) -Acetyl CoA (2 carbon molecule) oxidized to CO 2. Krebs Cycle intermediates return to initial state & generate NAD re from electrons liberated in the process. 3
Overview of Energy Extraction Chemoorganotrophy (autotrophs and heterotrophs) Glycolysis (anaerobic) 4
Krebs Cycle (aerobic) Respiratory Chain Phosphorylation Electron transfer ultimately reduces O 2 to H 2 O, but some of the energy liberated in the process is conserved as ATP (3 ATP produced per molecule of NAD re processed). Energy from the aerobic respiration of 1 glucose molecule: 2 ATP + 2 NADre 2 NADre 8 NADre 2 ATP + 12 NADre x3 ATP/NADre respiratory chain 2 ATP + 36ATP = 38 ATP/molecule glucose compare: 38 ATP/glucose - AEROBIC metabolism 2 ATP/glucose - ANAEROBIC metabolism Anaerobic organisms must process 19X more organic substrate to produce the same amount of ATP-energy as aerobic organisms. Aerobic organisms will operate efficiently at relatively dilute substrate levels. Anaerobic organisms have (by necessity) to be very simple and not very active to minimize energy expenditure (bacteria, yeast but also some protozoa, incl. endosymbiotic chemotrophic bacteria). Also inhabit environments with high substrate levels - e.g., decaying organic matter. 5
Oxidation of Organic Matter with Different e - Acceptors REACTION ΔG o (kcal/mole) CH 2 0 + O 2 CO 2 + H 2 O -686 5CHO+4NO 2 3-4 HCO 3- +CO 2 +3HO+2N 2 + 2-570 CH 2 O + 3 CO 2 + H 2 O + 2 MnO 2 4 HCO 3- + 2 Mn 2+ -349 CH 2 O + 7 CO 2 + 4 Fe(OH) 3 8 HC0 3- + 3 H 2 O + 4 Fe 2+ -114 2CH 2 0 + SO 4 2 HCO 3- + H 2 S -77 CH 2 0 CO 2 + CH 4-58 ΔG o (kcal/mole) = free energy released per mole of glucose oxidized CONCEPT: Some energetic transformations are more energetically favorable than others. These will usually occur first under natural conditions - i.e., the most energetically favorable terminal electron acceptor (O 2 ) will be used until it is no longer available, then the environment will favor organisms (bacteria) capable of utilizing alternative electron acceptor to oxidize organic matter. Biological Utilization of Chemical Energy 1. Energy Currency ATP - Economic analogy for the transformation of energy in the cell - need for a "medium of exchange". Most biochemical reaction series requires elaborate cell machinery and organization, and many specific enzymes. It is not efficient, and not possible, for enzyme complexes to handle all possible combinations of substrates, intermediates, and sources of energy. METABOLIC processes (e.g, respiration) "oxidize" organic molecules, capturing some of their energy in a single molecule which is recognized as the "energy donor" or "medium of exchange" in all subsequent reactions. This molecule, ATP, is special because it carries a fixed amount of energy in an easily released form high energy phosphate bonds. 6
2. Active Transport: work of moving molecules &ions against concentration gradients According to the 2nd Law of Thermodynamics - everything in universe tends toward increased entropy (randomness). Therefore, energy must be expended to bring things (e.g, molecules) into a more organized and concentrated state. Functions of active transport: 1. Provides proper chemical environment for cellular processes (e.g., ph). 2. Brings needed substrates (glucose, amino acids) &essential minerals (nitrate, phosphate, & important ions K+ and Ca++) where they are needed. 3. Gets rid of waste products (H+, Na+, C02, lactic acid). Characteristics of active transport: 1. A given systems is specific for a particular molecule or ion. 2. Transport occurs in a specific direction across membrane. Transport is accomplished by enzymes at "active sites" - i.e., substrate specific with specific orientation (directionality) in cell membrane matrix. 3. Powered by ATP molecules which "fit" into an active sites and donates high energy phosphate bonds to the process. 4. Works against continuous back diffusion (which occurs at a slower rate because it is not enzyme aided). Active transport is generally a continuous process in living cells, concentrations on either side of the membrane are maintained in "dynamic equilibrium" 2. Active Transport (cont.) 7